Pantheon, Rome

Built in 118 AD by Emperor Hadrian (the same guy that built Hadrian’s Wall in the north of England), much of it still exists intact today.  The columns weigh 60 tons each and were transported from Egypt to Rome on barges.  The temple was built to honor Pagan gods (the name is derived from the Ancient Greek: referring to all gods) and exists on the site of a former temple (built 27 BC) dedicated to the gods Mars and Venus.  In 609 it was converted to a Church.  The consecration of the building as a Church saved it from the destruction that befell most Ancient buildings during the Medieval period.